The present invention relates to a copy prevention technology for a digital video system, and more particularly, to a copy prevention method and apparatus for a digital VCR to which encryption is introduced to display a picture only in a VCR internally containing a corresponding encryption code, thereby preventing tape from being copied.
General copy prevention methods for analog VCR are presented in U.S. Pat. Nos. 4,819,098, 4,571,642 and 4,577,216.
First, U.S. Pat. NO. 4,819,098 discloses a method in which an interference signal is inserted into a video waveform in an automatic gain control circuit (AGC) of a VCR. Here, the inserted signal does not affect the AGC of its monitor but has the AGC of the VCR record an accurate level of signal on a video tape.
In U.S. Pat. No. 4,571,642  4,577,216, there is presented a method in which a phase noise or other corrected signal is inserted into the chrome  chroma burst of a video waveform.
However, all the conventional technologies insert a distributing signal to an analog signal using the difference between a circuit of a monitor and a corresponding circuit of a VCR. Some VCRs may perform copy normally despite of copy prevention. Some monitors cannot display images of the original video tape. A conventional copy prevention introduced to an analog VCR system is hard to be applied to digital storage media (DSM).
Specifically, in a satellite or high-definition TV decoder, as shown in FIG. 2, an MPEG bit stream received by a digital VCR is constructed to transmit a transport header, packetized elementary stream (PEG)  (PES) header and audio and video data respectively or simultaneously.
The PES header contains a PES header flag area of 14 bits which is a field for DSM such as digital VCR, and a PES header field having a variable length. The PES header flag area includes 1-bit copyright (CR) flag, 1-bit original-or-copy (OC) flag, 2-bit PD flag, 1-bit TM flag, and 1-bit AC flag.
The PES header field varies in length, and part thereof is set by the PC  PD, TM and AC flags. A PTS/DTS area is not present if the value of the PD flag is “00”. It is 40 bits if the value “10”. If the value is “11”, the area is 80 bits. A DSM trick mode field is not present if the TM flag is “0”. If the flag is “1”, the field is 8 bits. An additional copy information field is 8 bits if the AC flag is “1”.
When recording is carried out by the satellite receiver or high-definition TV decoder and compressed video data is encoded in encoder 101, it is converted into a packet form in packet processing portion 122  102 as shown in FIG. 1. If the compressed audio data is encoded in audio encoder 103, it is converted into a packet form in packet processing portion 104.
When the outputs of packet processing portions 102 and 104 are multiplexed in transmission multiplexer 105, a fixed transmission stream shown in FIG. 2 is output to a digital VCR. In this case, for copy prevention, a public-key encryption is applied which is suggested in U.S. Pat. No. 4,200,770. This solves disadvantages in key management or key distribution when a conventional block-cipher or stream cipher algorithm such as data encryption standard (DES) encrypts or decrypts only with a secret key.
This public-key encryption system has all users U hold unique encryption algorithm EPKU and description algorithm DPKU. Here, encryption algorithm EPKU for the public-key is opened as a public-key to key supply portion 107. Decryption algorithm DPKU for secret key is kept in secret. The characteristics of EPKU and DPKU are as follows.
First, with respect to all users U and message m transmitted, DPKU(EPKU(m))=m.
Second, encryption algorithm EPKU and decryption algorithm DPKU do not require complicated calculation.
Third, it is impossible to find DPK″U satisfying DPK″U (EPKU(m))=m from encryption algorithm EPKU.
In the encryption system having the above characteristics, as shown in FIG. 3, when user A transmits message m to user B, encryptor 106 receiving public-key algorithm EPKU for user B's public-key from key supply portion 107 encrypts message m (EPKU(m))=c) and transmits the result to decrypter 109 via a public channel. Here, the public channel indicates a channel in which transmitted data is not kept in secret.
Key decrypter 108 receiving the key information from key supply portion 107 outputs an algorithm DPKB, corresponding to encryption algorithm EPKB, decryptor 109 decrypts (DPKB(c))=m) the output of encrypter 106 with decryption algorithm DPKB, and then transmits to user B. In other words, only user B can decrypt decryption algorithm DPKB corresponding to encryption algorithm EPKB.
A concept developed from the public-key encryption is presented in U.S. Pat. No. 4,405,829. This public-key encryption system is called RSA system. A method in which the RSA public-key encryption is efficiently calculated via batch processing is presented in U.S. Pat. No. 4,964,164.
However, this public-key encryption is inappropriate for high-velocity encryption. A CA system is intended to present  prevent illegal view viewing. However, there is no method of protecting a program distributed through a digital storage medium such as a digital VCR.